This type of data collection system is mostly used for data collection from one or a plurality of terminal units each provided in a place difficult of access for an electric wave, such as basement of a building.
In a building, a power line is laid to an underground section thereof, so that, if each of the plurality of terminal units is connected to a data collector with a power line, the data collector can collect data via the power line from each of the terminal units.
In recent years, to achieve improvement of an S/N ratio or to realize high speed transmission in a transmission path with a power line, there have been proposed a data collection systems, in which data is superimposed on a power line by means of spread spectrum modulation and extracting the data from the power line by spread spectrum modulation.
A similar technology using spread spectrum communication is disclosed in, for instance, Japanese Patent Laid-Open Publication No. SHO 60-162326. Disclosed in this publication is a technology in which, in a case where data is transmitted from a terminal unit, the terminal unit checks a transmission state of each of other terminal units from a signal level on a power line and confirms that any of the terminal units does not transmit any data.
Although the technology disclosed in the publication is effective in preventing signals transmitted from each of the terminal units from colliding with any other signal on the identical transmission path, in turn the transmission cannot be made until it is detected that the transmission path is available.